Antimony trade group raises concerns over proposed threshold limit

Users would see increased costs and process issues

14 February 2019
/ Exposure scenarios, Metals, United States

The International Antimony Association (i2a) has said a recent proposal to lower the threshold limit value (TLV) for the substance antimony trioxide would cause production challenges and increased costs for many users.

Antimony substances are used extensively in flame retardants and also in lead batteries, alloys, plastics, paints, glass and other ceramics.

Earlier this month, US scientific organisation the Association Advancing Occupational and Environmental Health (ACGIH), recommended that the threshold limit for inhaling antimony trioxide should be 0.02mg/m³. The ACGIH’s recommended limit has been 0.5mg/m³ since 1979.

If the updated proposal is adopted, many users of the substance would face problems across their production processes, as well as higher costs, says i2a secretary general Caroline Braibant. While the ACGIH is not a regulatory agency, its opinions and conclusions on substances are well respected and can influence regulatory decisions.

To achieve such a limit, users of antimony trioxide would need to use low-dust or dust-free forms of the substance, such as wetted powders or masterbatches, which is where antimony is already added to a polymer, for example.

"Half of the production volume of antimony trioxide in the EU has already moved to these supply forms. But the cost of these forms to users is slightly higher because of the extra processing involved," says Ms Braibant. In addition, implementing the relevant workplace controls to comply with the threshold limit value may actually require significant, and much larger, investments, she adds.

The dust-free form of antimony, she says, is however "sometimes perceived as not as compatible with some downstream production processes".

Solutions to this issue can be established through better user and producer communication, says Ms Braibant. "Users can discuss the process barriers with their producers, explain their process needs and the producers may then be able to adjust."

But some users are not open to sharing their process details for confidentiality reasons.

"Therefore, they prefer to buy the pure powder form, despite the higher exposure potential it entails" she says.

‘Little detail’

In 2017, following the release of the US National Toxicology Program (NTP) Carcinogenicity studies on antimony trioxide, the ACGIH proposed a respirable limit of 0.03mg/m³. But the ACGIH withdrew the proposal after it held a public consultation.

ACGIH calculated its new proposed value for inhalable exposure by transforming the lowest concentration of respirable antimony – where the NTP studies’ mice and rats developed adverse chronic lung effects (3mg Sb/m³) – into a human equivalent concentration. It then divided it by a number of "uncertainty factors".

"This calculation mostly boils down to interpreting the very recent NTP toxicological animal evidence against the very limited reliable and relevant workplace exposure information available," an i2a press release says.

"However, very little detail is provided regarding the fashion in which respirable aerosol impacts in the NTP studies were converted to a proposed inhalable limit or the uncertainty factors that were applied in ACGIH’s derivation of the proposed TLV," it adds.

The difference between respirable and inhalable is that the former measures the particles that reach and enter the deep lung, while inhalable measures where they enter the nose and mouth.

In addition to submitting comments on the proposal, i2a plans to inform the ACGIH about its workplace monitoring programme, which kicks off next week.

"The data collected through this programme should replace a number of old, incomplete and unreliable workplace monitoring data ACGIH is currently referring to," the press release says.